RCA CTC4B "Gainsborough" picked up in San Francisco

[Electronic M]

Originally Posted by Tom9589

Think of the CRT and the shunt regulator as two variable loads on the flyback. When the CRT has a bright scene, it becomes a greater load while the shunt regulator becomes a lessor load. Conversely, a dark scene causes the CRT to become a lessor load while the shunt regulator becomes a greater load. The whole idea here is to present a more or less constant load on the flyback and the circuitry upstream of it. The real question is whether the designers specified the flyback so that it can withstand this combined load (CRT plus shunt regulator) on a continuous basis without having too much of a temperature rise.

That is a good explanation (and a good part of the point I was going to make before reading it). I'd like to add that the regulator is also an "emergency limiter"....if say the CRT heater goes out for some reason without the regulator in place there is no load on the HV winding of the flyback, and it then can easily shoot up to 30-40kV in a CTC-4....Once it hits that range the factory insulation on and INSIDE the HV winding is near to beyond it's design rating, and prolonged operation under those conditions is likely to cause external and INTERNAL arc over and all the damage that goes along with that.

Most late 60's sets with a pulse regulator also get their focus from a resistor divider fed from the CRT's HV lead, and probably also serve as an emergency minimum HV load.

Chris, one thing that now comes to mind is that you are calibrating the horizontal and HV system with that set on a test jig (right?), the jig likely is not providing the exact yoke (and, as mentioned, CRT) characteristics the chassis will see with it's own CRT and yoke that are in the cabinet. I doubt all those adjustments will still be correct once the chassis is back in it's cabinet, and I'd be inclined to redo them when the chassis is ready to be mounted. My cabinet acted as it's own test jig when I was servicing my set (cables have enough reach to allow the back of the chassis to face the cabinet and reach), and upon reflection I'm glad I did it that way.

[ohohyodafarted]

Good discussion guys! Now lets have some input on your theories as to the causes of flyback overheating in sets that have HV that is within acceptable limits.

It has been my experience that a transformer does not overheat unless it has an excessive load placed on it, or the transformer has shorted windings.

What do you fellows think? Are you going to point more to a defective flyback with a short or did something place an excessive load and that caused overheating and then a resultant short.

IF it is excessive loading, what was the most likely cause?

[ChrisW6ATV]

I appreciate this discussion of flyback and HV circuits, it is good to learn from others with more knowledge than I have with these early sets.

Originally Posted by Electronic M

Chris, one thing that now comes to mind is that you are calibrating the horizontal and HV system with that set on a test jig (right?)

Yes, I am using a jig, and you made some good points, thanks. I will indeed re-check everything once I move the chassis to the cabinet.

Since I had the overheating/meltdown/arcing with the set powered on for a relatively short time (20 minutes or so perhaps) compared to having it run an hour or more several times previously without a problem, I suspect that I caused the overheating by running the horizontal circuits off-frequency too long or too far, as I did the waveshaping adjustments.

I did a new set of tests today, including running the chassis longer and at 110-115 volts, with no problems seen. After being powered on for over 20 minutes, the flyback was just low to moderately warm, certainly not warm enough to cause wax to melt off if it was present, and no warmer at the arcing point than elsewhere, so it seems to have survived.

Bob, I am not sure why I was seeing that 120mA H output cathode current reading, and in fact today's reading was 80mA for a few minutes before going up to closer to 200mA while I was looking elsewhere during testing. I have to suspect my Simpson meter (maybe dirty contacts; I have never used its current function since restoring and testing it a year or more ago), since there was no change at all in the appearance of the picture before and after the displayed current change. I checked the current with and without a 0.1uF/630V capacitor across the meter connections, and I also tried my Fluke DMM with and without the capacitor. Neither meter showed any difference without the capacitor, so I will say it is not needed here as some of you already said was likely. The meter readings (all of this was after the Simpson readings "jumped", and they stayed consistent once they did that) were: Simpson ~195mA at ~108V line voltage (from the Variac) and ~205mA at ~115V, Fluke 188mA at ~108V and 198mA at ~115V. I did verify that the H linearity coil was set for lowest current (well, as soon as it went even a couple of mA higher, I stopped and went back, in both directions; I had already adjusted it for a bit lower current yesterday).

I will plan to do current readings again after I put the chassis in the cabinet, too.

[Electronic M]

Bob: You are ignoring door number 3: The makers designed the set so the flyback would run hot, and not have sufficient power headroom or cooling provisions to do otherwise....Many RCA designs did this or were on the edge to the point that non-optimal conditions would push them over (though IMHO the CTC-4 was one of their better H deflection/HV system designs).

Chris: May I suggest a procedure to select operating voltage? When I was working on my CTC-4 I set the line voltage such that the highest B+ rail directly derived from the power transformer matched the spec in the schematic.

[old_coot88]

Another trick using the variac is to put your AC voltmeter on the heater supply (preferably right where it comes out of the xfmr), and set the variac for exactly 6.3 VAC, then see what primary voltage is.

[ChrisW6ATV]

Thanks for the Variac suggestions. I will try that, both ways. One day, I want to make a few line-bucking transformer setups for my antique TV sets.

[old_coot88]

Congradulations. Looks like you dodged the bullet since the fly is running cool with no localized heating at the damage spot, and the K current is OK.

This should be an object lesson on why one should never, ever leave any vintage TV, BW or color, running unattended, even for a moment.

[miniman82]

Originally Posted by ohohyodafarted

Good discussion guys! Now lets have some input on your theories as to the causes of flyback overheating in sets that have HV that is within acceptable limits. It has been my experience that a transformer does not overheat unless it has an excessive load placed on it, or the transformer has shorted windings. What do you fellows think? Are you going to point more to a defective flyback with a short or did something place an excessive load and that caused overheating and then a resultant short. IF it is excessive loading, what was the most likely cause?

So you're asking the chicken/egg question? lol

The simple answer is we're working with 60-70 year old electronics at this point and it's a miracle any of it works anymore, but beyond that these are the culprits I've personally run into.

1. Running sets with higher line input voltage than called for in the service information. All the sets I've ever worked on were specified to accept 115-117VAC input from the wall, and the reality is these days we can have anywhere from 115 to 130VAC at the wall plug. There's not much regulation going on with the lines coming from the utilities, so during summer when everyone is running the AC it's usually on the low side and right now when it's cold at night time and demand is low it can get pretty high. This has unintended consequences with the horizontal circuits in a set but people don't think about it, because it doesn't affect anything else in their home because we now have switching supplies that account for it in the rest of our appliances.

Because you have a linear power supply with step up transformer in old TV's, the line drifting high causes the B+ supply to drift up as well. Only a few volts difference on the primary can cause 50+ volts extra to show up at the secondary, which then gets passed directly to the horizontal section and flyback transformer. Those extra volts will also cause a rise in horizontal cathode current all else being equal (the primary resistance of the flyback hasn't changed, so check ohm's law!), which means the flyback is being strained and it's natural to expect it to run hotter as a result.

Because of this, I refuse to run any of my sets without an autotransformer or variac and verifying AC input voltage.

2. Not having the horizontal section aligned correctly. This could be anything from a wave shaping coil set wrong to the HV regulator tube drawing excessive amounts of current, it all falls under the same banner. Some are content to simply go through the procedure in Sams and assume it's good, meanwhile it's running away with excessive current draw and next thing you know it melts down for seemingly no reason. I don't stop till I can put a number on HOT current, which I then annotate in the schematic as an accepted norm. This way if I ever mess with it again, I know what it was last time and can watch for changes.

3. Moisture. This one we can't do anything about so far as I know, but when sets sit all the components will be absorbing moisture from the environment. If said environment happens to be a damp/wet basement, watch out. It can infiltrate the windings of the flyback and cause the copper wires to corrode leading to shorts, intermittant pops, overheating, incorrect resistance readings, nearly anything you can think of within reason. I know shango dunnked a fly in hot wax and boiled out the moisture, but for the vast majority of us this will not be an option. Best thing you can do is actually run the set, because the heat from the flyback will drive the moisture out naturally if given enough time.

4. Failed CRT. Sometimes despite your best efforts a fly gets taken out by an arcing CRT, and you can't do anything about it. Sometimes parts fail, just the way it is. Doesn't have to be a reason which always sucks, but like I said 70 year old TV's...